Thread: Vector Iteration Problems

Hi. I've declared a vector and now I want to add 1 to every spot in it that comes up from a random number generator. So if the number 6 came up I would want to increment vector[6] by 1. I have code that looks like it should be working but gives an error everytime the program is run.

Code:

vector <unsigned> Dicegame::PlayGame (unsigned numPlays) 
{ 

        for (unsigned i = 0; i < myPlaysPerGame; i++) 
        { 
                unsigned j = TossNDice(); 
                myGameResult[j]++; 
        } 


        return myGameResult.size(); 
}

You should probably give more information about the error, and maybe more code (like how you initialze your vector). Also, please copy and paste your code when you post (if possible) because typos make it hard to distinguish the actual problem you are having.

Just like with a regular C-style array, you cannot access an element in the vector with [] unless an element actually exists (that index is not out of the bounds of the array). Your code is only safe if you know that TossNDice will only give you values within a particular range (e.g. 1-12) AND your vector has a size of one more than the highest number in that range (e.g. size of 13 because the indexes start at 0).

If you don't know the range that TossNDice will return, or if the range is too big to use a vector efficiently, try using a map. Your code will work properly as is.